1. Field of the Invention
The present invention relates to a lock mechanism for a webbing retractor employed in a seatbelt system for a vehicle designed to restrain the body of an occupant when an emergency situation of the vehicle occurs, the lock mechanism being adapted to prevent an occupant restraining webbing from being unwound at the time of such an emergency.
2. Description of the Related Art
In a typical seatbelt system for a vehicle, an occupant restraining webbing is wound up into a retractor from one end thereof by means of a predetermined biasing force, and a lock mechanism is incorporated in the retractor for the purpose of preventing the webbing from being unwound when an emergency situation of the vehicle occurs.
One type of such conventional lock mechanism has a lock wheel provided in such a manner that the lock wheel rotates together with the rotation of a webbing take-up shaft in a direction in which the webbing is unwound, and a resilient member interposed between the lock wheel and the take-up shaft. Thus, when the vehicle runs into an emergency situation, the lock wheel is stopped from rotating by the action of an acceleration senser means, and a lock means which rotates together with the take-up shaft is activated by the lock wheel the rotation of which is delayed with respect to the rotation of the take-up shaft so that the lock means is engaged with ratchet teeth formed on a frame.
This type of conventional lock mechanism suffers, however, from the following problems. When the vehicle is running on a rough road with many irregularities, the acceleration sensor means may be undesirably kept in an operative state. If so, the lock mechanism is unfavorably maintained in its locked state, which means that it is impossible for the occupant to unwind the webbing. Further, during such running on a rough road, the occupant's body is repeatedly bumped up and down by the vibration of the vehicle body, and the webbing is thereby repeatedly wound in and out. As a result, every time the webbing is wound in, the position of engagement between the lock member of the lock means and the ratchet teeth gradually advances, so that the webbing is gradually wound up into the retractor, which fact may unfavorably increase the pressure applied to the occupant's body from the webbing.
In order to overcome these problems, the applicant of the present invention has already proposed lock mechanisms for a webbing retractor so designed that, even when the webbing is repeatedly wound in and out by a given length, there is no fear of the webbing being gradually wound up into the retractor (see Japanese Utility Model Application Nos. 86496/1984, 122267/1984, 120834/1984 and 120835/1984).
In these proposed lock mechanisms, a rotary plate is provided in such a manner that the rotation of the take-up shaft is transmitted to the rotary plate through friction, and adapted to limit the movement of a pawl lever for actuating the lock means when the take-up shaft rotates in a direction in which the webbing is wound up, thereby eliminating the fear of the position of engagement between the lock member and the ratchet teeth advancing gradually even when the webbing is repeatedly wound in and out during the running on a rough road.
If the rotary plate, which is employed in this type of conventional lock mechanism in such a manner as to rotate in unison with the rotation of the take-up shaft, has a large diameter, a correspondingly large resistance acts against the rotation of the take-up shaft. It is therefore preferable to reduce the diameter and weight of the rotary plate. If the arrangement is such that a resilient member such as a leaf spring is mounted on the outer periphery of the take-up shaft so as to receive the rotational force of the take-up shaft by means of friction and this resilient member is disposed in opposing relation to the pawl lever for actuating the lock mechanism, it is possible to obtain a lock mechanism which has a reduced size and minimized inertia.
However, it is troublesome to machine or work a resilient member such as a leaf spring so that it has a configuration which is satisfactorily conformable with the outer periphery of the take-up shaft in order to bring the resilient member into close contact with said outer periphery, and it is difficult to bring such resilient member into close contact with the outer periphery of the take up shaft with a high degree of accuracy.